(1) Field of the Invention
The present invention relates to a wireless communication apparatus, and more particularly to a wireless communication apparatus suitable for use in a base station of an OFDM mobile wireless communication system.
(2) Description of Related Arts
In a digital mobile communication system in which a plurality of mobile stations are connected to a communication network via a base station, it is necessary for the base station to adjust reception timing of each upward frame transmitted from each of the mobile stations to reference timing of the base station. For this reason, when reception timing offset of the upward frame is detected, the base station performs time alignment (TA) control for requesting the mobile station to change the transmission timing.
With the increase in the number of accesses to the Internet from cellular phones and advance of digital processing technology in recent years, a high-speed and large-capacity communication scheme is required in a wireless section of the mobile communication system. OFDMA (Orthogonal Frequency Division Multiple Access) using OFDM (Orthogonal Frequency Division Multiplexing) modulation is noteworthy as a communication scheme satisfying such a requirement.
In OFDM, a multitude of subcarriers different in frequency from each other are used for modulating transmission symbols. In an OFDM transmitter, transmission data are converted into complex symbols in units of a predetermined number of bits by a modulator adopting, for example, QAM (Quadrature Amplitude Modulation), QPSK (Quadrature Phase shift Keying), and so forth. A sequence of complex symbols outputted from the modulator is inputted to an IFFT (Inversed Fast Fourier Transform) unit. The IFFT unit converts a predetermined number of complex symbols having been subjected to serial-parallel conversion into a plurality of complex numbers to be sample values (samples) of OFDM symbols. A plurality of samples forming the OFDM symbols are subjected to parallel-serial conversion and outputted as serial complex baseband OFDM signals from the IFFT unit.
Because a wireless signal transmitted from a transmitter arrives at a receiver, in a separated form as an indirect wave (delayed wave) reflected by an obstacle present on a propagation path and a direct wave unaffected by the obstacle, the receiver needs means for eliminating the influence of the indirect wave.
For that reason, an OFDM transmitter makes copies of a predetermined number of samples extracted from the trailer part of each OFDM symbol and inserts them as a guard interval (GI) in front of the OFDM symbol. The complex baseband OFDM signals having been inserted the GI are inputted to an RF unit and converted into analog serial signals by a D/A converter. The analog serial signals is superimposed on a carrier wave, and transmitted as RF band OFDM signal amplified in power. In an OFDM receiver, the GI is removed from the baseband OFDM signals outputted from an RF unit, and an FFT (Fast Fourier Transform) unit regenerates the original complex symbols by performing serial-parallel conversion, FFT (Fast Fourier Transform) conversion and parallel-serial conversion on the baseband OFDM signals.
In an OFDMA mobile communication system, subcarriers f0 to fn to be used in modulation of transmission data are shared by a plurality of users. The subcarriers f0 to fn are divided into a plurality of sub-channels each comprising a group of subcarriers. Multiple accesses is realized by allocating any one of the sub-channels to each user in time division manner. In the mobile communication system adopting OFDMA, the base station multiplexes communications for a plurality of users in a frequency region.
In the wireless communication using IFFT and FFT, such as OFDMA described above, communication quality deteriorates due to interference between the symbols when an FFT window deviates from a correct position on the receiving station side. For this reason, it is necessary to adjust the position of FFT window according to the time alignment (TA), as proposed in, for example, “A time and frequency synchronization scheme for multiuser OFDM” by J. J. van de Beek, P. O. Borjesson, M. L. Boucheret, D. Landstrom, J. M. Arenas, P. Odling, C. Ostberg, M. Wahlqvist, and S. K. Wilson, IEEE J. Select. Areas Common; vol. 17, pp 1900-1914, November, 1999.
In a cellular mobile communication system, in order to make effective use of frequency resources, frequencies are spatially reused in such a manner that adjacent base stations use carrier frequencies different from each other, while distant base stations use the same carrier frequency. Further, as a technology for avoiding interference between terminal stations being communicating with the same frequency, there is known an adaptive array antenna (AAA) technology that combines reception signals received by a plurality of antennas after weighting with different weights. In the AAA system, adaptive control is adopted so that the reception characteristic of antenna indicates null in the direction of an interference signal and directivity in the direction of a desired signal.
As an AAA combining unit adopted in the OFDMA wireless communication system, there are a Pre-FFT type that performs weighting of reception signals from antennas before FFT processing, and a Post-FFT type that performs weighting for each subcarrier after the reception signals are subjected to the FFT processing.
The Pre-FFT type AAA combining unit can reduce the number of weights to be generated, but it needs to estimate delayed waves in order to generate the weights. Therefore, Pre-FFT type has a drawback such that the antenna characteristic deteriorates when an incoming angle of interference waves spreads. On the other hand, since the Post-FFT type AAA combining unit does not need to estimate the delayed waves although the number of times of calculating weights increases, an excellent antenna characteristic is available even when an incoming angle of interference waves spreads.
The base station in the OFDMA wireless communication system has to adjust transmission timing of upward signals from all terminal stations by the time alignment (TA) control so that transmission signals from respective terminal stations (mobile stations) in a communication state can be received at the FFT window position by the base station. If the transmission timing of upward signal from one of the mobile stations deviates from the timing designated by a TA control signal from the base station, the upward signal received by the base station deviates from the FFT window.
In the case where a reception signal in a time region is expressed by f(t), and a reception signal in a frequency region by g(f), for example, a relationship between f(t) and g(f) can be represented by following expression in DFT (Discrete Fourier Transform):DFT[f(t)]=g(f)  (1)
Further, on the assumption that the number of sampling points (samples) of DFT is L, and the reception timing of upward signal at the base station deviates by τ samples from the reference timing of FFT window, following expression can be hold:
                              DFT          ⁡                      [                          f              ⁡                              (                                  t                  -                  τ                                )                                      ]                          =                              exp            ⁡                          (                                                -                  j                                ⁢                                                      2                    ⁢                    πτ                    ⁢                                                                                  ⁢                    f                                    L                                            )                                ⁢                      g            ⁡                          (              f              )                                                          (        2        )            
Here, j indicates an imaginary number, and exp( ) an exponential function. These expressions means that, in the case of orthogonal modulation signals, deviation (offset) of reception signal in the time region will show up as phase shift proportional to frequency in the frequency region. Accordingly, if FFT is performed in a state where a reception signal deviates from the FFT window, the FFT unit outputs the reception signal in a state as subjected to phase shift, regardless of the state of delay on the propagation path.
It is assumed that, for example, OPSK signals received by a first antenna and a second antenna are at signal point constellations in the frequency region as shown in FIGS. 17A and 17B, respectively, provided that no delayed wave exists. In this case, if the FFT window has no deviation, the signal point constellation of combined reception signal in the frequency region should be in a state as shown in FIG. 18. Although signal points (x marks) exist for each subcarrier, 16 signal points appear in the figure because a plurality of x marks overlap with each other. If the reception timing of upward signal deviates by one sample from the FFT window, as the signal point offset appears for each subcarrier, the signal point constellation of the combined reception signal in the frequency region will become, for example, in a state as shown in FIG. 19.